SHANG Mengya ,MIAO Wei ,JIA Dongqing ,CHENG Wenxi ,ZHANG Xin ,WANG Qiang ,LIN Haowei,LI Jinling＊

(1. School of Material Science &Engineering, Henan University of Technology, Zhengzhou 450001, China; 2. Henan International Joint Laboratory of Nano-photoelectric Magnetic Materials, Henan University of Technology, Zhengzhou 450001, China)

Abstract: In order to improve the mechanical properties and thermal conductivity of polymethyl methacrylate (PMMA),multi-walled carbon nanotubes (MWCNTs) were used as reinforcement through in situ polymerization method to prepare PMMA/MWCNTs composites by changing the reaction time,polymerization temperature and the content of MWCNTs.The effects of different reaction conditions on the properties of the composites were studied.The results show that the mechanical properties,thermal/electrical conductivity and thermal stability of the composites are improved compared with the PMMA matrix.The tensile strength of the composites is increased by up to 24%.The bending strength of the composite material increases from 20.41 to 68.04 MPa,and the maximum increase is 233%.Meanwhile,when the content of MWCNTs is 3 wt%,the thermal conductivity of the composite is 0.335 W/(m·K),which increases by 138%,and the electrical conductivity is 3.94 S/m.The thermal stability of the composite has been significantly enhanced.The modified PMMA will be widely used in medicine,communications,electronics and other fields.

Key words: polymethyl methacrylate;carbon nanotubes;mechanical property;thermal conductivity;thermal stability

1 Introduction

Polymethyl methacrylate (PMMA),commonly known as plexiglass,has the characteristics of high light transmittance (90%-92%) and good weather resistance[1-5],which can be used widely in window glass,daylighting materials and greenhouse ceiling.However,due to the extremely low crystallinity,poor mechanical strength and thermal conductivity,PMMA needs to be modified in order to improve its comprehensive performance.Due to the strongly interactingδcovalent bond,multi-walled carbon nanotubes (MWCNTs)[6]has excellent mechanical and thermodynamic properties due to its stable chemical bond and excellent structure,making it a better choice for enhancing PMMA performance.At present,studies on PMMA/MWCNTs composites mainly focus on improving the strength and toughness of PMMA matrix by using the excellent mechanical properties of MWCNTs.Meanwhile,MWCNTs with good thermal conductivity can be used as a modifier to improve the thermal conductivity of PMMA matrix greatly[7-10].Russelle[11]found that when the addition amount of multi-walled carbon nanotubes (MWCNTs) to PMMA was between 0.1% and 10%,the dispersion effect of MWCNTs was preferable.In recent years,with the development of PMMA modification and composite material technology,the application fields of PMMA are constantly expanding,such as medical polymer materials,optical guide fiber,military glass,etc.

In this article,in situpolymerization method was used to prepare PMMA/MWCNTs composites.Firstly,MMA monomer and acidified MWCNTs were dispersed uniformly by ultrasonic treatment.Then,by adjusting the reaction temperature,reaction duration and the content of MWCNTs,PMMA/MWCNTs composites were prepared by solution casting method using 2,2-azobisisobutyronitrile (AIBN) as initiator.Finally,the structure and properties of the composite materials were tested and analyzed by means of structural analysis,mechanical analysis,thermal/electric conductivity analysis and thermal stability test.

2 Experimental

2.1 Materials

MWCNTs (multi-walled,purity＞98%),MMA(analytical pure),AIBN (analytical pure),concentrated H2SO4(98%),concentrated HNO3(68%),acetone and absolute alcohol were purchased from Shanghai Sinopharm Chemical Reagents Co.,LTD.

2.2 Acidification of MWCNTs

Since MWCNTs tend to agglomerate during the polymerization,they should be acidified before use[12].10 mL of HNO3(68%) was put into a 250 mL beaker,then 10 mL of H2SO4(98%) was poured slowly along the beaker wall and stirred evenly to prepare mixed acid[13].A certain amount of MWCNTs were weighed and placed in a 250 mL flask.The mixed acid was slowly poured into the flask and shaken well.The mixed solution was dispersed by ultrasonic for 30 min,then the MWCNTs were centrifuged,washed with absolute alcohol several times and dried at 50 ℃ in a vacuum oven for 12 h.

2.3 Preparation of PMMA/MWCNTs composites

Taking the mass fraction of MWCNTs was 0.3 wt% as an example,0.084 g of MWCNTs were added into 30 mL of MMA monomer and dispersed by ultrasonic for 35 min.Then the mixed solution was placed in an oil bath with vigorous stirring and the temperature was settled to 90 ℃.When the temperature was above 70 ℃,0.05 g of AIBN was added into the solution.After 20 min,the viscosity of the prepolymer was measured every 5 min to observe the viscosity change of the system to avoid the explosive polymerization.During the reaction process,observe whether there was a drawing phenomenon.When the drawing phenomenon formed in the polymerization system,the pre-polymerization reached the end point,and the resulting sample was taken out and cooled to room temperature with ice water.The sample was transferred into a mold,cured at 80 ℃ in a vacuum oven for 4 h,and demoulded at room temperature.On this basis,by changing reaction time (15,30,45,60 and 90 min),polymerization temperature (80,85 and 90 ℃) and the content of MWCNTs (0.1 wt%,0.3 wt%,0.7 wt%,1 wt%,2 wt% and 3 wt%),contrast experiments as well as the corresponding characterization work were carried out.Among them,when the effect of reaction conditions was studied,the content of MWCNTs was 0.3 wt%.When the effect of the content of MWCNTs was studied,the reaction time was 40 min and the reaction temperature was 85 ℃.Pure PMMA and PMMA/MWCNTs composites were made into rectangular samples with a size of 60 mm ×13 mm × 2 mm on a plate vulcanizing press,polished with sandpaper for tensile/bending properties test.

2.4 Characterization

The Scanning electron microscope (SEM)images were acquired with a Inspectf50 (American FEI Company).The Fourier transform infrared spectroscopy (FTIR) spectrum was examined on an IR Prestige-21 (Shimadzu,Japan).X-ray diffraction (XRD)analysis was carried out on a Bruck D8 Advance diffractometer using Cu Kα radiation (λ=0.154 05 nm,40 kV,100 mA).Mechanical properties were tested on a universal testing machine (WDW-10,Yongkang),and the tensile speed was 5 mm/min and the specimen span was 24 mm with a bending speed of 2 mm/min.Impact properties were obtained on a film impact tester (TY-4021A,Tianhui),and the impact sample height was 40 mm with a speed of 3.5 m/s.The thermal conductivity was obtained through transient hot-wire method (DREIII,Xiangyi).Electrical conductivities were measured by a Hand-held LCR digital bridge (Victor4080,Yisheng Shengli).Thermal stability of the samples was tested by a thermogravimetric analyzer (WCT-2C,Jingyi Gaoke),and the temperature range was 50-600℃ with a heating increment of 10 ℃/min.

3 Results and discussion

3.1 Characterization of PMMA/MWCNTs composite materials

Fig.1 shows the morphology of cross sections of pure PMMA and PMMA/MWCNTs composites.It can be seen from Fig.1(a) that pure PMMA matrix has poor continuity.As shown in Fig.1(b)-1(d),when the content of MWCNTs increases from 0.3 wt% to 3 wt%,the cross-section of PMMA/MWCNTs composites become smooth,the PMMA matrix at the cross-section is better wrapped,and the continuity of the composites becomes better,indicating that through ultrasonic dispersion andin-situpolymerization,MWCNTs disperse in the PMMA matrix uniformly,and realize good combination within the matrix,thus achieving the result of strengthening the matrix material.

Fig.1 SEM images of (a) PMMA;(b) PMMA/MWCNTs (0.3 wt%);(c,d) PMMA/MWCNTs (3 wt%)

The chemical composition changes during the polymerization of PMMA/MWCNTs composites were carefully investigated.As shown in Fig.2(a),for PMMA matrix,the C-H symmetric stretching vibration absorption peak of -CH2-is appeared near 2 995 and 2 953 cm-1.The absorption peak near 1 747 cm-1is strong,corresponding to the stretching vibration peak of C=O.The absorption peak near 1 382 cm-1is the C-H symmetric stretching vibration peak of -CH3.There are many weak shoulder peaks in 1 300-1 600 cm-1regions.The stretching vibration absorption peak of C-C skeleton is near 1 193 cm-1.The absorption peaks near 1 144 and 1 065 cm-1correspond to stretching vibration of C-O.FT-IR spectra of PMMA/MWCNTs composites obtained at different reaction times and polymerization temperatures are shown in Fig.2(b)-2(c),which present similar absorption peak of PMMA matrix.The stretching vibration peaks of C=O appear near 1 701,1 747 and 2 250 cm-1.The C-H symmetric stretching vibration peak of -CH3is near 1 370 cm-1.In addition,the addition of MWCNTs make the C-H symmetric stretching vibration absorption peak of the -CH2-in PMMA near 2 995 and 2 953 cm-1disappeared.Meanwhile,the weak shoulder peak in the low wave number region weakens or even disappears.

Fig.2 FT-IR spectra of (a) pure PMMA;PMMA/MWCNTs composites obtained at different (b) reaction time;(c) polymerization temperature;(d) content of MWCNTs

FT-IR spectra of PMMA/MWCNTs composites with different contents of MWCNTs are shown in Fig.2(d).When the content of MWCNTs is lower than 0.7 wt%,the strong and characteristic bond of PMMA/MWCNTs at 1 156 and 1 250 cm-1do not change,and the peak of the C=O double bond near 1 740 cm-1do not disappear.Therefore,when the content of MWCNTs is low,the chemical bonds of PMMA/MWCNTs are similar to that of PMMA matrix,but the addition of MWCNTs weakens part of the light energy,leading to the decrease of the light transmittance of the PMMA matrix.The addition of MWCNTs weakens the strong and characteristic band of PMMA/MWCNTs.However,when the content of MWCNTs is higher than 1 wt%,C-O characteristic bond still appears at 1 150 and 1 190 cm-1,and a pair of double peaks appear near 1 250 and 1 270 cm-1.Compared with PMMA,the characteristic band still appears and the strong bond near 1 730 cm-1do not change,so the addition of MWCNTs has not changed the chemical bonds in the matrix,but affects the strength of each absorption peak[14-16].

The XRD spectra of PMMA and PMMA/MWCNTs were analyzed (Fig.3).The characteristic absorption peak of MWCNTs appears at 2θ=14.05° and the peak of PMMA/MWCNTs (0.3 wt%) is stronger than that of PMMA/MWCNTs (0.7 wt%),indicating that MWCNTs are better dispersed at lower content.When the content of MWCNTs is higher than 1 wt%,a new characteristic peak appears at 2θ=26.48°,and the higher the content of MWCNTs is,the stronger the characteristic peak is.This peak is not the characteristic peak of MWCNTs,and we infer that it is a new phase state generated when the content of MWCNTs reaches a certain proportion and compounds with PMMA matrix,which may produce different properties of composites.For example,when the content of MWCNTs reaches the critical value and enters the conductive percolation zone,the matrix material interacts with MWCNTs and forms conductive path,therefore the conductivity of the composites increases exponentially.Through conductivity data,it can be seen that the conductivity of PMMA/MWCNTs changes dramatically when the content of MWCNTs is higher than 0.7 wt%,which is consistent with the appearance of new characteristic peak in the XRD spectra which is inferred as a conductive phase of PMMA/MWCNTs (1 wt%)[17,18].PMMA/MWCNTs has not reach the end zone of conductive percolation when the content of MWCNTs is 3 wt%,indicating that the content of MWCNTs is still the development stage of conductive pathway in the range of 1 wt%-3 wt%,and this new peak is stronger when the content of MWCNTs continues to increase.

Fig.3 XRD spectra of PMMA/MWCNTs composites with different contents of MWCNTs

3.2 Tensile properties of PMMA/MWCNTs composites

Tensile properties of different PMMA/MWCNTs samples are shown in Fig.4.The tensile strength of pure PMMA is 24.56 MPa.With the addition of MWCNTs,due to the higher surface energy of MWCNTs,the binding force between MWCNTs and PMMA matrix is enhanced,which improves the rigidity and toughens the PMMA matrix.When the reaction time is 30 min,the tensile strength reaches 30.43 MPa.When the reaction time is prolonged,the tensile strength decreases at first and then increases,as shown in Fig.4(a),due to the agglomerate of MWCNTs when the viscosity of the reaction system increases.Meanwhile,when the prepolymer is poured into the mold,pores and defects appear in the samples,and stress concentration is easy to occur during stretching,which affects the mechanical properties finally.The influence of polymerization temperature on the tensile properties of PMMA/MWCNTs is shown in Fig.4(b).When the reaction temperature rises,AIBN can open the π bond of MWCNTs and induce the polymerization of MMA monomer on the surface of MWCNTs.MWCNTs are fully combined with the polymer,oriented and distributed evenly in the PMMA matrix.The interaction between them is enhanced and the load is transferred to MWCNTs which have high tensile strength and elastic modulus,so the tensile strength of PMMA/MWCNTs is improved.When the polymerization temperature is 90 ℃,the tensile strength of PMMA/MWCNTs is 29.85 MPa.The influence of the contents of MWCNTs on the tensile strength of PMMA/MWCNTs is shown in Fig.4(c).When the content of MWCNTs is lower than 0.3 wt%,MWCNTs with surface effect and small size effect act as an enhancer inside the composites.The tensile strength of PMMA/MWCNTs increases with the increasement of content of MWCNTs.When the content of MWCNTs reaches 0.3 wt%,the tensile strength of PMMA/MWCNTs is 27.01 MPa.When the content of MWCNTs is higher than 0.3 wt%,the enhancement effect of MWCNTs on the tensile strength of the matrix is significantly weakened,or even lower than that of the PMMA matrix,due to the dispersion of MWCNTs in the polymer matrix decreasing with the gradual increase of the content of MWCNTs.Agglomeration occurs,which leads to stress concentration in some areas and reduces the tensile strength of PMMA/MWCNTs[19].

3.3 Bending properties of PMMA/MWCNTs composites

The bending strength of different PMMA/MWCNTs composites can be investigated in Fig.5.The bending strength of pure PMMA is 20.41 MPa.The strong binding force between MWCNTs and PMMA matrix improves the bending strength of PMMA/MWCNTs to a certain extent.When the reaction time is 30 min,the maximum bending strength of PMMA/MWCNTs is 68.04 MPa.With the extension of the prepolymer time,the prepolymer is transferred to mold without vacuum treatment,resulting in more bubbles and defects in the sample,so bending strength decreases after 30 min.Temperature is an important factor affecting the rate of polymerization reaction.When the reaction temperature rises,the rate of polymerization and various side reactions accelerates,as shown in Fig.5(a).The influence of different polymerization temperatures on the bending properties of PMMA/MWCNTs is shown in Fig.5(b).When the polymerization temperature is 85 ℃,the maximum bending strength of PMMA/MWCNTs is 67.59 MPa.At 90 ℃,the accelerated reaction rate of the system leads to branching and even cross-linking reaction of some macromolecules.The dispersion of MWCNTs in the polymerization system is affected thus the bending strength is reduced.The bending strength of PMMA/MWCNTs with different contents of MWCNTs are shown in Fig.5(c).When the content of MWCNTs is low,MWCNTs disperses well in the matrix,and the bending strength of PMMA/MWCNTs increase with the increasement of the content of MWCNTs.When the content of MWCNTs is 0.1 wt%,the maximum bending strength of PMMA/MWCNTs is 47.45 MPa.When the content of MWCNTs exceeds 0.1 wt%,the dispersion of MWCNTs become poor and PMMA/MWCNTs is subjected to uniform stress,and the bending strength of the composites decreases with the increasement of the content of MWCNTs.

Fig.5 The bending properties of PMMA/MWCNTs composites obtained at different (a) reaction time;(b) polymerization temperature;(c) content of MWCNTs

3.4 Impact properties of PMMA/MWCNTs composites

Impact properties of different PMMA/MWCNTs samples are shown in Fig.6.The impact strength of pure PMMA is 1.44 J/mm2.With the addition of reinforced MWCNTs,MWCNTs with high toughness are dispersed in PMMA matrix.The interfacial bonding strength of PMMA/MWCNTs composites is enhanced,the glass transition temperature is reduced,and the impact strength is increased.With the prolongation of reaction time,the impact strength increases at first,then decreases after reaching the highest point,and becomes stable finally (Fig.6(a)).When the reaction time is 30 min,the impact strength reaches at a maximum of 1.58 J/mm2.The influence of polymerization temperature on the impact properties of PMMA/MWCNTs is shown in Fig.6(b).When the polymerization temperature is 80 ℃,the maximum impact strength is 1.77 J/mm2.When the temperature rises gradually,MWCNTs play the role as strengthening phase,but the particle size of PMMA/MWCNTs composites increases,damaging the structure of the polymer,therefore the impact property of PMMA/MWCNTs is decreased.The impact properties of PMMA/MWCNTs with different contents of MWCNTs are shown in Fig.6(c).In the range of 0.1 wt%-3 wt%,the impact strength of PMMA/MWCNTs decrease by 11.8%,9.3%,7.4%,6.1% and 5.2%,respectively,compared with PMMA matrix,for the reason that MWCNTs with highly aggregated structure are difficult to disperse and distribute in the matrix,and the dispersion direction of MWCNTs in the matrix leads to its anisotropic mechanical properties.

Fig.6 Impact properties of PMMA/MWCNTs composites obtained at different (a) reaction time;(b) polymerization temperature;(c) content of MWCNTs

3.5 Thermal conductivity of PMMA/MWCNTs composites

To investigate the thermal conductivity of PMMA/MWCNTs composites,two samples with a radius of 39 mm and a thickness of 2 mm were prepared.The hotwire probe was fixed in the middle of the two samples,the test time was set at 160 s,and the output power was 0.25 W.PMMA is a semi-crystalline polymer,the main carrier of its heat conduction is phonons,and the thermal conductivity is only 0.141 5 W/(m·K).MWCNTs have high thermal conductivity,and act as nucleating agent in the polymerization process to promote the crystallization of PMMA and enhance the thermal conductivity.Fig.7(a) shows the thermal conductivity of PMMA/MWCNTs prepared at different reaction time.With the increase of polymerization time,MWCNTs gradually aggregate with MMA monomer,and the interfacial thermal resistance decrease.The thermal conductivities of the composites obtained at 30,45,60 and 90 min are 0.259 7,0.242 78,0.250 2 and 0.298 1 W/(m·K),respectively.Compared with pure PMMA,they increase by 83.6%,71.6%,76.8% and 110.7%,respectively.Fig.7(b) shows the thermal conductivities of PMMA/MWCNTs prepared at different polymerization temperatures,which are 0.253 4,0.244 8 and 0.233 2 W/(m·K),respectively.Compared with pure PMMA,the thermal conductivities of PMMA/MWCNTs increase by 79.1%,73.0% and 64.8%,respectively.Thermal conductivity of PMMA/MWCNTs with different content of MWCNTs increases with the increasement of the content of MWCNTs,as shown in Fig.7(c).When the content of MWCNTs increases from 0.1wt% to 3wt%,phonon transfer efficiency is enhanced,and corresponding thermal conductivity increases from 0.168 to 0.335 W/(m·K),which improves by 15.5% and 137.7% compared with pure PMMA,respectively.

Fig.7 Thermal conductivity of PMMA/MWCNTs composites obtained at different (a) reaction time;(b) polymerization temperature;(c) content of MWCNTs

3.6 Electrical conductivity of PMMA/MWCNTs composites

Samples were made with the size of 10 mm × 10 mm × 3.8 mm to measure the resistance value (R).The resistivity of the sample was calculated by using the formulaρ=RS/L,and then the conductivity was obtained by usingδ=1/ρ.The cross-sectional areaSwas 3.8×10-5m2of the measured sample,and the lengthLis 0.01 m.The conductivities of PMMA/MWCNTs with different contents of MWCNTs are shown in Fig.8.According to the result,as the content of MWCNTs increases from 0 to 3 wt%,the conductivities of the composites are 1.54×10-4,9.7×10-4,0.067,0.69,2.31 and 3.94 S/m,respectively.When the content of MWCNTs is in the range of 0-0.3 wt%,the conductivity of PMMA/MWCNTs is not much different from that of pure PMMA.When the content of MWCNTs is higher than 0.7 wt%,the content of conductive parts in PMMA/MWCNTs reaches a critical content,and the composite system begins to enter a conductive percolation zone,where the conductive path changes from onedimensional,two-dimensional to three-dimensional.When the content of MWCNTs is in the range of 0.7 wt%-3 wt%,the conductivity of PMMA/MWCNTs increases exponentially,which can be explained by the conductive percolation theory of polymer matrix composites[20].In PMMA/MWCNTs composites,when the content of MWCNTs is low,conductive parts do not contact each other and conductive pathways are not formed,and the material is usually characterized by high resistance characteristics of PMMA matrix at this time.But with the increase of the content of MWCNTs,conducting parts begin to connect each other in the PMMA matrix,thus conductive path in the composites formed,and the electrical conductivity of PMMA/MWCNTs mutates at this time.In the following conductive percolation zone,the electrical conductivity of PMMA/MWCNTs shows exponential growth until reaching the end zone of conductive percolation,where the content of MWCNTs has no or little effect on the electrical conductivity of the composites.

Fig.8 Influence of content of MWCNTs on the conductivity of composites

3.7 Thermal stability of PMMA/MWCNTs composite

The thermal stability of PMMA/MWCNTs was studied through thermogravimetry (TG).The addition of MWCNTs plays as a nucleation role in the polymerization process,and MWCNTs disperse in the crystallization region,which increase the thermal decomposition temperature of PMMA/MWCNTs by about 20 ℃,significantly improving the thermal stability,and the weight loss rate of the composites is lower than that of pure PMMA[21].Due to the higher thermal conductivity of PMMA/MWCNTs,the composite materials are more likely to decompose when heated,so it can be seen from the Fig.9 that the weight loss of PMMA/MWCNTs occurs earlier than that of PMMA matrix.

4 Conclusions

PMMA/MWCNTs composites are successfully synthesized byin-situpolymerization method.With the addition of MWCNTs,the maximum stress and mechanical properties of the composites are improved.

Compared with pure PMMA,the tensile strength,bending strength and impact strength are increased to 30.43 MPa,68.04 MPa,1.77 J mm-2,which increase by 24%,233% and 23%,respectively.

The addition of MWCNTs can significantly improve the thermal and electrical conductivity of PMMA,and the higher the content of MWCNTs is,the more obvious the enhancement effect on the thermal conductivity of PMMA matrix is.

The thermal stability of PMMA/MWCNTs has been significantly enhanced,with the thermal decomposition temperature increasing by about 20 ℃.

In summary,the optimal polymerization time of PMMA/MWCNTs composites is 30 min,and the optimal reaction temperature is 80-85 ℃.When the content of MWCNTs is 1 wt%,the PMMA/MWCNTs composite not only has good mechanical properties,but also has high thermal/electrical conductivity,which expands the application area of PMMA in the fields of thermal and electrical conduction.

Conflict of interest

All authors declare that there are no competing interests.